Dr. Andreas Wýrsig is Vice Head of the Department for Integrated Power Systems at the Fraunhofer-Institute for Silicon Technology ISiT in Germany. Automotive IQ spoke with Dr. Andreas Wýrsig about innovative materials for car batteries and the challenge of producing the volume of batteries the automobile industry actually requires.

What is your background and how long have you been with the Fraunhofer Institute?

I studied chemistry at the University of Freiberg, in Saxony starting in 1996. After that I went to Switzerland where I did my PhD from 2002 – 2005 under the supervision of Prof. Petr Novak. The main topic of this work was the characterization of lithium ion batteries with methods like DEMS and XRD. From 2005 to 2006 I worked as a post graduate at the University of Basel on sensor technology. In 2006 I changed to the Fraunhofer Institute ISIT in Itzehoe were I am now vice head of the department "Integrated Power Systems".

What materials do you see as having the best pros vs. cons in terms of using them for car batteries? Are there any materials that are particularly innovative that you see coming to production in the next few years?

It depends on the type of the car. If we are talking about a full electric vehicle, the energy density is most important. For the anode, carbon based active materials like graphite are the materials of choice. New materials with higher energy density are currently developed using silicon as the main component. A major obstacle is the large volume expansion of silicon during charging. This leads very quickly to cracks in the structure and amorphisation of the material, so you can cycle it only a limited number of times.

The application of nano sized Si can provide a solution and great progress has already been made. I think Si will come to be used in commercial cells in a relatively short period of time, perhaps two to three years in my opinion. That timeline was also mentioned by bigger advisory companies. As cathode material we see nickel manganese oxide (NMC) in many of the commercials cells. It also has a high energy density and compared to other materials it is rather safe. In the near future we will see the introduction of cathode materials with increased nickel content and with higher charging voltage. There are already some high voltage electrolytes available. This will further increase the energy density and lower the price. I think NMC will be the material of choice for the car industry for the coming years.

For hybrid vehicles or vehicles that need some kind of a booster, meaning an energy-power-peak for a short period time the power density of the battery becomes more important. In this case new materials like lithium titanate (LTO) can be used. LTO can provide a very high cycle stability and durability, combined with very good power density. Test results with 15,000 cycles were published for LTO and a lifetime of about 20 years can be expected. In addition it is very safe. The material is not flammable and therefore does not provide additional energy in case the cell catches fire. Another advantage is it’s the low temperature

behaviour.

A graphite based lithium accumulator if charged at temperatures below 0° Celsius can develop a short circuit because metallic-lithium can be plated onto the surface of the graphite. This is not uniform plating, but lithium grows in the form of dendrites that can perforate the separator. A lot of problems can come from this. Lithium-titanate, on the other hand, does not have this behaviour. The major drawback is that the energy density of LTO is rather low because the nominal discharge voltage with cathode material

like NMC is around 2.3 volts, instead of 3.7/3.8 and it also has a rather low specific capacity with about 160 mAh/g instead of about 360 mAh/g for graphite. In stationary applications, there aren’t any big issues with space and weight. That’s why it is used by several companies in that sphere already. We have been working on lithium-titanate for several years now, so far mainly for stationary applications. A good cathode material for high power applications is lithium iron phosphate (LFP). It is already in the market for several years. It has very good safety features e.g. is not prone to thermal runaway but also has a lower

energy density compared with materials like NMC.

With graphite, cells could short circuit because they would essentially contact each other?

Yes. In this case you have metallic-lithium-dendrite that grows from the anode side through the cathode side, which leads to a short circuit in the cell. So the temperature rises in the cell and if you have cathode material, which is prone to thermal runaway, you start to ignite the whole cell. It’s one of the drawbacks of the graphite-based systems. Of course, there are a lot of measures you can take against this, but this reduces the possibility of using it at low temperatures.

Do you see any market factors limiting this, i.e. availability?

So far they are all available. There have been some discussions about availability of materials like cobalt, nickel and lithium. Many were on the lookout for lithium sources. But there is enough lithium supply around.

I understand that production is incredibly challenging and that only a handful of manufacturers can produce in the volume the automobile industry requires. Can you discuss this challenge?

That’s really a big issue. I mean more and more big cells are being produced for electric vehicles. Big, I mean above 20 Ah, maybe with the exception of Tesla, who are still using 18650 cells. I think for, the moment, it’s quite a good way to do it, because the development is very good. They have a high energy density today and have a very good quality, because they have been producing them for many years and the producers normally know what they are doing.

One of the biggest issues for quality control is: If you use material like electrode-foil or separator-foil and it has undetected damages and you use it to build, let’s say 3 Ah 18650 cells, you may have to throw away 2-3% of your production. If you have the same amount of failures in 40 Ah cells, you may have to throw away 40% of your production, for the same failure-rate. That’s because you need much bigger areas of electrode-foils and separator-foils. And this is really one of the issues the manufacturers have at the moment, as they need very good quality control during the manufacturing process. There are solutions for that, but still the output of the bigger cells is not as good as of 18650 cells.

Do you think that in the next 4-5 years the number of major players able to produce cells are going to remain the same? Or do you see some new players entering?

That depends on the development of the electro-mobility market. There were high hopes some years ago that this market is going to grow substantially up until 2020. Now, however, we see that the growth is not progressing in this fashion. But if the market growth comes back, maybe because of the increasing number of electric vehicles that are coming into the market now, I do think that we are going to see new manufacturers, but maybe from bigger companies that decide to build their own cell manufacturing.

I don’t think that there are going to be smaller companies with a completely new introduction into the market, especially in Europe. Rather established players looking for new markets. For other fields of applications, like stationary application, the possibility that smaller manufacturers might come into play is much higher. This is because the safety issues for stationary systems are easier to meet and entering into that market is probably cheaper. We have seen some of them in Germany too, but some of them are also struggling very hard at the moment in this volatile market. The bigger players have difficulties as well. This is in part because of the price drop that we have seen in comparison to previous years. In China you can already buy cells now for 160-180 $/kWh, if not for an application in cars. Only 3 years ago these prices were predicted for well after 2020.

The price drop could be a good thing for consumers eventually.

Yes. The price has to go down, so the electro mobility market can have the big growth that everybody is expecting. But I think that the price drop happened too fast for the market players to reduce the costs and stay competitive.

What kind of safety strategies for cells do the OEM’s demand?

In the cells you have to differentiate between pouch cells and hard-case cells like prismatic or cylindrical cells. Pouch cells are not being used very much in the market for electric vehicles at the moment. Normally they have a higher energy density but in case of a cell failure they have a tendency to swell and to crack open quicker than hard case cells do. To increase the safety, intrinsic measures like flame retardants and shuttle additives can be introduced for both type of cells. For hard-case cells pressure relief valves are very important, so in case of over pressure, the valves open and the gas is able to get out of the cells and thereby prevent the cell from exploding at some point.

A one-way membrane?

Yes. It’s a one-way membrane. You can throw the cell away after it is open. But at least it doesn’t explode. And usually there is an overcurrent protection or a fuse, which stops the current flow in case of a failure. The fuse is usually a kind of polymer. There are also extrinsic measures you can take and especially the cell monitoring and the battery management system has to be designed very carefully in order to prevent the cells from getting out of the current range or voltage range they are designed to be.

Do you see any chance that active balancing techniques are going to come down in costs?

Many are hoping that active balancing becomes cheaper. I have seen some developments going in this direction from various universities and several institutes. I think that we can see a price drop for active balancing in the near future. But it is difficult to predict where the prices will be.

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